Turbine blades (stator blades and rotor blades) of aircraft engines, industrial gas turbines and other systems are often operated in high-temperature environments for a prolonged time and thus are made of a Ni-based single crystal superalloy that has an excellent heat resistance. The Ni-based single crystal superalloy is produced in the following manner. Al is first added to base Ni to cause Ni3Al to precipitate for precipitation strengthening. High melting point metals, such as Cr, W and Ta, are then added to form an alloy which is formed as a single crystal. The Ni-based single crystal superalloy acquires a metal structure suitable for strengthening through solution heat treatment at a predetermined temperature and subsequent aging heat treatment. The superalloy is called a precipitation hardened alloy which has a crystal structure with a precipitation phase (i.e., γ′ phase) dispersed and precipitated in a matrix (i.e., γ phase).
As the Ni-based single crystal superalloy, a first generation superalloy contains no Re at all, a second generation superalloy contains about 3 wt % of Re, and a third generation superalloy contains 5 wt % or more to 6 wt % or less of Re, have been developed. The superalloys of later generations acquire enhanced creep strength. For example, the first generation Ni-based single crystal superalloy is CMSX-2 (Cannon-Muskegon Corporation, refer to Patent Document 1), the second generation Ni-based single crystal superalloy is CMSX-4 (Cannon Muskegon Corporation, refer to Patent Document 2) and the third generation Ni-based single crystal superalloy is CMSX-10 (Cannon Muskegon Corporation, refer to Patent Document 3).
The purpose of the third generation Ni-based single crystal superalloy, CMSX-10, is to enhance creep strength in high-temperature environments as compared to the second generation Ni-based single crystal superalloy. The third generation Ni-based single crystal superalloy, however, has a high composition ratio of Re of 5 wt % or more, which exceeds the solid solubility limit with respect to the matrix (γ phase) of Re. The excess Re may combine with other elements in high-temperature environments and as a result, a so-called TCP (topologically close packed) phase to may precipitate. A turbine blade incorporating the third generation Ni-based single crystal superalloy may acquire an increased amount of the TCP phase when operated for a prolonged time in high-temperature environments, which may impair creep strength.
In order to solve these problems, a Ni-based single crystal superalloy having higher strength in high-temperature environments has been developed. In such a superalloy, Ru for controlling the TCP phase is added and the composition ratios of other component elements are set to optimal ranges so as to provide the optimal lattice constant of the matrix (γ phase) and the optimal lattice constant of the precipitate (γ′ phase).
Namely, a fourth generation Ni-based single crystal superalloy which contains about 3 wt % of Ru and a fifth generation Ni-based single crystal superalloy which contains 4 wt % or more of Ru have been developed. The superalloys of later generations acquire enhanced creep strength. For example, an exemplary fourth generation Ni-based single crystal superalloy is TMS-138 (National Institute for Materials Science (NIMS) and IHI Corporation, refer to Patent Document 4), and an exemplary fifth generation Ni-based single crystal superalloy is TMS-162 (NIMS and IHI Corporation, refer to Patent Document 5).
The fourth and fifth generation Ni-based single crystal superalloys, however, include a large amount of heavy metal such as W and Re, in order to enhance the creep strength in high-temperature environments, and thus have a high specific gravity as compared to the first and second generation Ni-based single crystal superalloys. As a result, a turbine blade incorporating the fourth or fifth generation Ni-based single crystal superalloy is excellent in strength in high-temperature environments, however, since the weight of the blade is increased, there are problems that the circumferential speed of the turbine blade may be decreased and the weight of the aircraft engine and the industrial gas turbine may be increased.
In order to solve these problems, a Ni-based single crystal superalloy which has a low specific gravity as compared to the fourth and fifth generation Ni-based single crystal superalloys although its creep strength is high in high-temperature environments has been developed by specifying a composition range of W to optimal ranges suitable for keeping excellent creep strength in high-temperature environments and by specifying a composition range suitable for structural stability, with reducing an amount of W which has a high specific gravity (refer to Patent Document 6).
Furthermore, in recent years, a Ni-based single crystal superalloy which has a high composition ratio of Re as compared to the above-described conventional Ni-based single crystal superalloys (the composition ratio of Re is more than 8 wt % in the concrete) has been developed (refer to Non-Patent Document 1). This Ni-based single crystal superalloy is called as a high-rhenium single crystal Ni-base superalloy in the Non-Patent Document 1 and includes 9 wt % of Re in the composition ratio as shown in Table 1 of the Non-Patent Document 1.